1. Field of the Invention
The present invention relates to an objective lens system for optical recording type disks, such as digitally recorded audio disks, video disks, etc. .
2. Description of the Prior Art
In such objective lens system, it is required to correct the aberrations to an extraordinarily high degree and to achieve a high resolving power, in order to form, on the disk, an extreme small spot image whose size is determined in accordance with the limit of the diffraction of light. Thus, a numerical aperture about 0.4.about.0.5 is necessary in type of lens system.
An objective lens system of this type has been proposed, for example, in Japanese Laid-Open Patent Application No. SHO No. 55-4068. However, a conventional optical recording type disk device is constructed as shown in FIG. 1. In FIG. 1, a light bundle emitted from a laser light source (2) is converted to a parallel light bundle by a collimating lens system (4). The parallel light bundle is incident on a beam splitter (6) from the light source side. The beam splitter (6) has a half mirror for dividing the light incident thereon by reflection and transmission. Thus, the parallel light bundle transmitted through the beam splitter (6) is concentrated into a spot on a rear surface of disk (10) by an objective lens system (8). The reflected light bundle from the disk (10) is incident on the beam splitter (6) from the disk side through the objective lens system (8). One part of such light bundle is reflected on the beam splitter (6) toward a lens system for focus detection (12), and another part thereof is transmitted through the beam splitter (6). The light bundle reflected toward the lens system (12) is incident on a light receiving device (14) through the lens system (12).
The lens system (12) is used for detecting the focusing condition of the objective lens system (8) on the rear surface of the disk (10). This lens system (12) includes a cylindrical lens element (12a) in order to generate a great amount of astigmatism in a predetermined direction perpendicular to the optical axis. The light receiving device includes plurality of light receiving cells which are arranged symmetrically with respect to the optical axis. Thus, the focusing condition of the objective lens system is detected in accordance with the output signals from such light receiving cells. A diffraction grating (16) is arranged so that the tracking condition of the light bundle incident on the disk (10) is detected a well-known three beam method.
FIG. 2 shows another arrangement of the prior art in which the collimating lens system (4) is arranged on the disk side of the beam splitter (6).
In the above arrangement, two methods for tracking of the light bundle incident on the disk (10) are known. One of the methods is to use a galvano mirror which is vibrated for changing the position of the light spot on the disk (10). However, in this method, it is necessary to correct various aberrations of the objective lens system with respect to the off-axial rays, since the light rays incident on the objective lens system would have some inclination angle to its optical axis. Thus, it causes to increase the number of the lens elements in the objective lens system, and to deteriorate the compactness of the objective lens system due to the extension of the focal length thereof.
Therefore, another method is often applied. This method is to shift the objective lens system perpendicularly to its optical axis for tracking of the light bundle. This method has an advantage in that it requires to correct the aberrations of the objective lens system only with respect to the paraxial rays, because the light rays incident on the objective lens system are always parallel to its optical axis. However, in this method, the construction of the objective lens system is required to be light in weight and to be simple, because the objective lens system itself is shifted for tracking.
Furthermore, it should be raised to make a pick-up device for disks more compact. Two methods would be considered for making the optical system in the pick-up device more compact. One of them is to shorten the focal length of the objective lens system for decrease the distance between the laser light source and the disk. However, the radius of curvature of any refractive surface in the objective lens system become small in proportion to the shortening of the focal length of the objective lens system. The sensitivity of the optical performance to a manufacturing error is undesirably increased in proportion to shortening of the focal length of the objective lens system. Thus, this method has a limit in minimizing the focal length of the objective lens system.
Another method is to increase the numerical aperture NA of the collimating lens system. However, this method requires the designer to increase the number of the lens elements in the collimating lens system. Thus, it causes and increase in the cost. Furthermore, in this method, the dispersion of the astigmatic difference in the laser light source becomes another problem.